1. Field of the Invention
The present invention relates to the capacitive displacement measurement technology, and more particularly to an absolute position measurement capacitive grating displacement measurement method, a sensor, and an operating method thereof.
2. Related Art
A capacitive grating displacement sensor is widely applied in the fields of linear/angular displacement measurement due to the characteristics thereof such as a low cost, a small size, and low power consumption. In terms of the realization principle, current capacitive grating displacement sensors are categorized into relative position measurement (an incremental type) capacitive grating displacement sensors and absolute position measurement (an absolute type) capacitive grating displacement sensors. The incremental type capacitive grating displacement sensor has been applied for about thirty years, and in this kind of sensors, a displacement amount needs to be rapidly accumulated, resulting in two defects: a measurement speed limit (lower than 1.5 m/s @150 KHz) and uninterruptible measurement (so further decrease of a working current is limited), such that the incremental type capacitive grating displacement sensor is gradually phased out. The absolute type capacitive grating displacement sensor is initiated by a Japanese company named Mitutoyo, and the implementation method thereof may be referred to patents CN89106051, U.S. Pat. No. 5,053,715, CN92101246, and CN93117701. This kind of sensor uses two or more code channels (wavelengths) to perform absolute positioning, so the requirement for rapid accumulation of a displacement amount is eliminated, and the work is in an intermittent measurement state (approximately 8 times per second), thereby overcoming the main defects of the incremental measurement. However, the existing absolute type capacitive grating displacement sensor has the following defects.
1. A sensor drive signal is a static (irrelevant to time) and spatially distributed waveform. A demodulated received signal is irrelevant to time (a direct current (DC) signal), and a harmonic effect cannot be alleviated through signal processing technologies.
2. In order to downsize a harmonic component, a sine waveform electrode that is difficult to make is required to be used.
3. Analog-to-digital (A/D) conversion of two orthogonal signals is required to be performed to determine a displacement amount in each wavelength.
4. An arctg operation is required to be performed to determine a displacement amount in each wavelength, which exceeds a real-time processing capability of a common Micro Controller Unit (MCU). In order to reduce a processing load of the MCU, linear approximation has to be adopted.
5. In order to reduce a linear approximation error, a sensor drive signal is required to be probed repeatedly to make a received signal close to a zero point.
6. In order to reduce the linear approximation error and a harmonic component effect, a relatively small fine wavelength (1.024 mm when the resolution is 0.01 mm) is required to be used.
7. The determination of the displacement amounts in all wavelengths hampers and influences each other, and during rapid movement, the algorithm does not converge.
In conclusion, the existing absolute type capacitive grating displacement sensor requires an MCU to be a core, software is based on an inefficient probing method, and peripheries require support of technologies such as complicated A/D conversion and a sine waveform electrode. Although a regular Single Chip Microcomputer (SCM) can indeed meet the aforementioned requirements on software and hardware, it is not easy to integrally install the system (made into a single-chip Application-Specific Integrated Circuit (ASIC)) on a handheld measurement tool to obtain a product that has a low cost, a small volume, low power consumption, and is suitable for mass production at the same time.
Patent ZL200710050658 introduces a round grating sensor for absolute position measurement, and the patented solution is applicable to angle measurement in a large distributed space. But an SCM is used for performing secondary processing on measurement results of two independent incremental type capacitive grating systems, which makes the cost, size, and power consumption several times as those of a normal incremental type capacitive grating system. Although functions of the incremental type capacitive grating system are extended, but inherent defects of the incremental type measurement are not solved.